1. Field of the Invention
The present invention relates to a fixing mechanism for fixing the ink discharged onto a recording medium, and an ink jet recording apparatus using the fixing mechanism.
2. Related Background Art
If a recording sheet just recorded with an ink jet recording apparatus, on which the ink has not been completely dried up, is touched, an image may be smeared or the ink may stick to the hands. With the ink jet recording apparatus having a high recording speed, a second recording sheet may be stained on its back face if it is laid over a first recording sheet before the ink of the first recording sheet dries. Therefore, among ink jet recording apparatuses, a high speed recording apparatus having a full-line type recording head requires means for drying and fixing the ink onto a recording sheet.
Especially, in a facsimile apparatus with a short distance from a recording unit to a paper exhaust port, a fixing mechanism is important. As the means for drying and fixing the ink, a thermal fixing method is conventionally used in which a recording sheet just recorded is rapidly heated to a high temperature. In the ink jet recording, it is commonly practiced to dry up all the ink in such a manner as to provide a fixing unit on a back side of a record face of the recording sheet in which a surface heater having a width exceeding that of the recording sheet is placed in contact with an entire surface of the recording sheet, because an image may be spoiled if the record face is rubbed before the ink has not been completely dried up. Besides, other methods are known in which the radiant heat is applied onto the recording sheet from above (Japanese Patent Application No. 59-209148), or the fixing feature is provided on the platen (Japanese Patent Application No. 1-285352).
Among the above-mentioned fixing methods, a generally practiced method in which a plate-like or surface heater is placed in contact with a back side of the recording sheet is carried out in such a manner that the record face is pressed against the surface heater at a very small contact area such as a plurality of points or lines, so that the heat of the surface heater can be efficiently transmitted to the recording sheet.
In the conventional examples as above described, the surface heater is placed in contact with an entire face of the recording sheet, but if the surface heater is disposed to be in contact with a recording sheet just recorded and a high temperature is abruptly applied to thermally fix the ink, in order to make the apparatus smaller, there is a problem that the ink may be dried up before ink droplets can permeate the recording sheet sufficiently, so that the diameter of dot may be smaller. Also, there is a problem that the non-uniformity of density may be produced as the density is partially increased on the upstream side in the conveyance direction (on the side entering a fixing unit from behind). Therefore, it is necessary to dispose the surface heater a certain distance away from the recording section, which impedes the accomplishment of a smaller apparatus.
Further, there is a problem that when a quantity of ink droplets is discharged, wrinkles or non-uniform densities may be produced on the recording sheet if the thermal fixing is made with the abrupt application of a high temperature mediately after recording.